In recent years, the internal combustion engine of an automobile gasoline engine and the like has improved in fuel efficiency taking environmental issues into consideration. Accordingly, engine specifications have trended toward increased compression so as to make it possible to exhibit a higher thermal efficiency and extract greater kinetic energy. The automobile gasoline engine, having a pressure for pressing down the piston that increases in proportion to the compression ratio, tends to have a high output and a high torque compared to an engine having a low compression ratio under identical volumes of exhaust and input fuel. However, in the automobile gasoline engine, an increase in compression ratio results in an increase in a temperature and a pressure inside a combustion chamber at a top dead center of the piston, causing a mixture of fuel and air to ignite before appropriately formed. As a result, in an automobile gasoline engine having a high compression ratio, combustion occurs locally inside the combustion chamber, making the combustion of fuel near the top dead center of the piston difficult, causing knocking. In this case, the automobile gasoline engine can no longer achieve the desired output or torque, and is more susceptible to nitrogen oxide (Nox) generation as well as soot production.
Further, examples of measures used to appropriately achieve combustion near the top dead center of the piston in the automobile gasoline engine include decreasing the temperature of a combustion chamber wall. Here, to decrease the temperature of this combustion chamber wall, it is effective to decrease the temperature of a piston crown surface that directly receives an explosion pressure in association with a reciprocating motion of the piston. Examples of means for decreasing the temperature of the piston crown surface include means for improving a heat transfer function required in a piston ring mounted to the piston, whereby a combustion heat received by the piston is efficiently transmitted to a cylinder, making it possible to appropriately achieve combustion near the top dead center of the piston. From the above, the piston ring used in the automobile gasoline engine having a high compression ratio particularly requires further improvement in a gas seal function and the heat transfer function.
With such a background, Patent Document 1 (Japanese Laid-Open Patent Application No. 2009-235561) proposes a piston ring that specifies suitable component ranges for carbon (C), silicon (Si), manganese (Mn), and chromium (Cr) under predetermined parameters. Specifically, the piston ring of Patent Document 1 comprises a heat-treated steel including C in a range of from 0.20 to 0.90 mass %, Si in a range of from 0.10 to less than 0.60 mass %, Mn in a range of from 0.20 to 1.50 mass %, Cr in a range of from 0.30 to 2.00 mass %, and a remnant being iron (Fe) and unavoidable impurities, wherein, based on contents of C, Si, Mn, and Cr, values of parameters A and B calculated from a formula “A=8.8 Si+1.6 Mn+1.7 Cr” and a formula “B=36 C+4.2 Si+3.8 Mn+4.5 Cr” are 9.0 or less and 10.8 or more, respectively (refer to claim 1).